In general, functional surface materials (FSMs) may be broadly described as materials which exhibit a reversible response in response to a stimuli. The study of a variety of FSMs has attracted a great deal of attention due to the potential for exploitation of these reversible changes in a variety of uses. One good example of a FSM is poly(N-isopropylacrylamide) (PNIPAAm) which exhibits a shift from being hydrophobic to hydrophilic, or from hydrophilic to hydrophobic, in response to changes in temperature. As such, surfaces modified by PNIPAAm have attracted a great deal of research attention because of the potential for the application of the characteristics of PNIPAAm, such as molecular switching of the surface by altering interfacial properties. See Dagani, R. Chem. Eng. News, 1997, June 9, 27, and Snowdown, M.; Murray, M.; Chowdry, B. Chemistry and Industry, 1996, July 15, 531. Such uses are possible because PNIPAAm exhibits a lower critical solution temperature (LCST), and remarkable hydration-dehydration changes in response to relatively small changes in temperature, when placed in an aqueous solution. See Heskins, M.; Guillet, J. E.; James, E. J. Macromol. Sci. Chem., 1968, A2,1441. Below the LCST, PNIPAAm chains hydrate to form an expanded structure, and above LCST, PNIPAAm chains dehydrate to form a shrinkage structure. This property is due to the reversible formation and cleavage of the hydrogen bonds between the NH or C.dbd.O groups of PNIPAAm and the surrounding water molecules brought about with changes in the temperature. See Okubo, M.; Ahmad, H. Colloid Polym. Sci., 1995, 73, 817. Since PNIPAAm's physical properties are readily controlled by simply changing temperature, and without changing the chemical structure of the polymer, a broad range of potential uses for temperature-responsive PNIPAAm have been suggested. Among other uses, PNIPAAM can be employed in drug delivery systems, see Hoffman, A. S. J. Controlled Release, 1987, 6, 297, for solute separation, see Feil, H.; Bae, Y. H.; Jan, F.; Kim, S. W. J. Membrane Sci., 1991, 64, 283, in the concentration of dilute solutions, see Trank, S. J.; Johnson, D. W.; Cussler, E. L. Food Technol., 1989, June, 79, for the immobilization of enzymes, see Dong, L. C.; Hoffman, A. C. J. Controlled Release, 1986, 4, 223, for the coupling of biomolecules, see Matsukata, M.; Takei, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. J. Biochem., 1994, 116, 682, and for the preparation of photosensitive materials, see Suzuki, A.; Tanaka, T.; Nature, 1990, 346, 345.
In one example where PNIPAAm has been bonded to a substrate by using plasma-treated polystyrene dishes grafted with PNIPAAm, the alteration of the hydrophilic/hydrophobic properties of the surface was observed as a response to temperature change. See Okano, T.; Yamada, N.; Okuhara, M.; Sakai, H.; Sakurai. Y. Biomaterials, 1995, 16, 297. In this study, endothelial cells and hepatocytes attached and proliferated on the PNIPAAm grafted surface at 37.degree. C., above the LCST of PNIPAAm. The cultured cells were readily detached from these surfaces by simply lowering the incubation temperature and without the usual damage associated with trypsinization. The radiation grafting utilized for this study had the advantage of being able to bind convalently the N-isopropylacrylamide (NIPAAm) monomer onto a chemically inert surface without contamination by potentially hazardous catalyst fragments. However, a problem remains due to the inconvenience and expense of radiation machines and damage to the materials from radiation, especially damage to the polymer materials.
In another study involving a glass substrate, temperature sensitive surfaces were prepared by coupling either PNIPAAm with a terminal carboxyl end group or random copolymers of PNIPAAm and acrylic acid, with the amino group on the glass surface by a water soluble carbodiimide, such as 1-ethyl-3-(3-dimethyamino-propyl)carbodiimide hydrochloride. See Okano, T.; Kikuchi, A.; Sakurai, Y.; Takei, Y.; Ogata, N. J. Controlled Release, 1995, 36, 125. In this study, each PNIPAAm-grafted surface showed completely hydrophilic nature below 20.degree. C. and a hydrophobic nature above the critical temperature. The coupling of amine and carboxyl groups involves the intermediary formation of the activated O-acylurea derivative of the carbodiimide. A subsequent nucleophilic attack by the primary nitrogen of the amino compound brings about the formation of the amide linkage with release of the soluble substituted urea. The formation of O-acylurea occurs optimally at pH 4-5. The intermediate has an extremely short life and rapidly undergoes hydrolysis or gives the N-acylurea adduct. The primary amino group of the nucleophile is predominantly protonated at this low pH and is rather unreactive. One the other hand, since it is an inhomogeneous reaction system, that is, the fact that reaction between the amine groups on the glass surface competes with the carboxyl groups on the polymer chains in solution, increases the difficulty of reaction. This limitation can severally restrict the yield of product under a variety of conditions. See Sehgal, D.; Vijay, I. K. Analytical Biochemistry, 1994, 218, 87.
Hydrogels, or water-swollen polymer gels, are one type of FSM which have attracted a great deal of attention in both theoretical studies and for real applications. See D. DeRossi, K. Kajiwara, Y. Osada and A. Yamauchi, Ed., Polymer Gels, Plenum Press, New York, 1989; P. S. Russo, Ed., Reversible Polymeric Gels and Related Systems, ACS Symposium Series 350, ACS, Washington, DC., 1987; and N. A. Peppas, Ed., Hydrogels in Medicine and Pharmacy, Vol. 1, CRC Press, Boca Raton, Fla., 1986. These polymer gels can be divided into two kinds, those which do not exhibit significant sensitivity to environmental changes, and those which change their properties in response to a variety of environmental stimuli including pH, See K. Kataoka, H. Koyo and T. Tsuruta, Macromolecules, 28, 3336 (1995); and S. Nishi and T. Kotaka, Macromolecules, 19, 978 (1986), temperature, see Y. Kaneko, K. Sakai, A. Kikuchi, R. Yoshida, Y. Sakurai and T. Okano, Macromolecules, 28, 7717 (1995); and T. Aoki, Y. Nagao, K. Sanui, N. Ogata, A. Kikuchi, Y. Sakurai, K. Kataoka and T. Okano, Polymer. J, 28, 371 (1996); photo, A. Suzuki and T. Tanaka, Nature, 346, 345 (1990); and A. Fissi, O. Pieroni, G. Ruggeri and F. Ciardelli, Macromolecules, 28, 302 (1995), pressure, See D. W. Urry, L. C. Hayes, T. M. Parker, R. D. Harris, Chem. Phys. Lett., 201, 336 (1993), and electrical fields, See T. Tanaka, I. Nishio, S. T. Sun and S. U. Nishio, Science, 29, 218 (1982). Among these, the temperature sensitive polymer gel, poly(N-isopropylacryamide) (PNIPAAm) has been of great interest because PNIPAAm demonstrates a lower critical solution temperature (LCST) and the temperature-dependent characteristics. See R. Dagani, Chem. Eng. News, June, 27 (1997); H. G. Schild, Prog. Polym. Sci., 17, 163 (1992); and M. Heskins, J. E. Guillet and E. James, J. Macromol. Sci., Chem., A2 (8), 1441 (1968). It swells with an extended chain conformation in aqueous solution below 32 C. and deswells with a compact chain conformation in aqueous solutions above 32 C. The phenomenon is caused by reverse formation and cleavage of the hydrogen bond between water molecules and hydrophobic molecular groups of PNIPAAm. The pentagonal water structure is suggested to be generated among water molecules adjacent to the hydrophobic molecular groups of PIPAAm. See D. W. Urry, Scientific American, January, 64 (1995). Since the pentagonal structure is stable at low temperature and unstable at high temperature, the reverse swelling-deswelling process can be observed with the variation of environmental temperature. As the volume phase transition brings about dramatic changes in the physical properties of the PNIPAAm gels, PNIPAAm and its copolymer gels are expected to be applied as the new types of materials, for example as actuators, See M. Snowden, M. Murray and B. Z. Chowdry, Chemistry and Industry, 15, 531 (1996), as temperature-modulated bioconjugaters to control enzyme activity, See L. C. Dong and A. S. Hoffman, J. Controlled Release, 4, 223 (1986) and as separation modules to extract water from the solution of macromolecules. See R. F. S. Freitas and E. L. Cussler, Chemical Engineering Science, 42, 97 (1987). Stimuli-responsive PNIPAAm gels have promising potential to achieve intelligent drug delivery system because it can be utilized as molecular device for self regulating drug delivery. See J. Riccka and T. Tanaka, Macromolecules, 17, 2916 (1984); A. Gutowska, Y. H. Bae, H. Jacobs, J. Feijen and S. W. Kim, Macromolecules, 27, 4167 (1994); S. Shoemaker, A. S. Hoffman and J. H. Priest, Appl. Biochem. Biotechnol, 15, 11 (1987) and H. Kurahashi and S. Furusaki, J. Chem. Eng. of Japan, 26, 89 (1993). One disadvantage of PNIPAAm gel is the poor mechanical properties. When the gels are fully swollen and absorbed by large amounts of water, the gels become unstable and are easily damaged by the effects of small stresses.
Surfaces modified by PNIPAAm to achieve the characteristics of hydrophobic/hydrophilic reverse change have also received a great deal of attention. See H. Iwata, M. Odata, Y. Uyama, H. Amemiya and Y. Ikada, J. membrane Sci., 55, 119 (1991); Y. M. Lee, S. Y. Ihm, J. K. Shim, J. H. Kim, C. S. Cho and Y. K. Sung, Polymer, 36, 81 (1995); and H. Kubota, N. Nagaoka, R. Katakai, M. Yoshida, H. Omichi and Y. Hata, J. Appl. Polym. Sci., 51, 925 (1994). Most of research work is concerned with the formation of thin PNIPAAm layer on the substrates by chemical grafting, See T. Okano, N. Yamada, H. Sakai and Y. Sakurai, J. Biomed. Mater. Res., 27, 1243 (1993), and plasma, See Y. G. Takei, T. Aoki, K. Sanui, N. Ogata, Y. Sakuarai and T. Okano, Macromolecules, 27, 6163 (1994). Such surfaces modified by PNIPAAm layer exhibit the reverse change of hydrophobic/hydrophilic properties and can be used in the incubation process of cells. See T. Okano, A. Kikuchi, Y. Sakurai, Y. Takei and N. Ogata, J. Controlled Release, 36, 125 (1995).
While prior art methods of attaching FSMs to substrates have some utility, many potential applications which would exploit the variable properties of FSMs require improved methods for attaching FSMs to substrates. Similarly, improved methods of attaching FSMs to substrates make possible new uses for FSMs in applications in microtechnology and for the use of FSMs as membranes or coatings used in the prevention of fouling, or anti-fouling. As such, there exists a need for improved methods for bonding FSMs to substrates and a need for new applications for FSMs in microtechnology and for anti-fouling.